1. Field of the Invention
The present invention relates to metal oxide catalyst carrier particles, a method for producing such particles, an exhaust gas purification catalyst made of the metal oxide catalyst carrier particles, and a method for recovering the exhaust gas purification catalyst.
2. Description of the Related Art
Exhaust gas from an internal combustion engine, such as an engine of an automobile, contains nitrogen oxide (NOx), carbon monoxide (CO), hydro carbon (HC), etc. These substances can be removed by an exhaust gas purification catalyst that is capable of oxidizing CO and HC and reducing NOx. A typical example of such an exhaust gas purification catalyst is a three-way catalyst that is made of a porous metal oxide carrier, such as γ-alumina, and precious metal, such as platinum (Pt), rhodium (Rh), and palladium (Pd), which is supported on the porous metal oxide carrier.
Usually, platinum is used as precious metal for three-way catalysts. However, when using platinum, there is a problem that, if exposed to a high-temperature exhaust gas for a prolonged period of time, platinum particles grow in size and thus the specific surface area of each platinum particle decreases accordingly, lowering the activation level of the catalyst. To counter this, various methods have been developed to suppress such growth of platinum particles.
For example, it has been discovered that, because ceria has a high affinity for precious metals exhaust gas purification, especially platinum, when ceria is used as the catalyst carrier, the growth (sintering) of precious metal particles is suppressed. In order to utilize this characteristics of ceria while compensating for disadvantages of using ceria having a relatively low heat resistance, JP-A-2005-313024, JP-A-2005-314133, JP-A-2004-141833, and JP-A-2005-254047 each describe using, as a carrier for platinum as the catalyst metal, metal oxide particles each having an outer skin portion containing much ceria and a center portion containing much alumina that is a metal oxide having a high heat resistance.
As a method for obtaining a metal oxide particle having an outer skin portion and a center portion that are different in composition, JP-A-2005-314133 describes a method in which: a sol containing colloid particles of a first metal oxide and colloid particles of a second metal oxide which have a different isoelectric point from that of the colloid particles of the first metal oxide is prepared; the pH of the sol is adjusted to be closer to the isoelectric point of the colloid particles of the first metal oxide than it is to the isoelectric point of the colloid particles of the second metal oxide so that the colloid particles of the first metal oxide are coagulated; the pH of the sol is then adjusted to be closer to the isoelectric point of the colloid particles of the second metal oxide than it is to the isoelectric point of the colloid particles of the first metal oxide so that the colloid particles of the second metal oxide are coagulated; and the coagulated product is then dried and calcined.
Meanwhile, JP-A-2000-202309 and JP-A-2003-74334 each describe a method for redispersing platinum particles, which have grown in size in use, on the carrier. The platinum redispersing method described in JP-A-2000-202309 heats exhaust gas purification catalysts, each comprised of a carrier, such as alkali earth metal oxide and rare earth oxide, and platinum supported on the carrier, at 500° C. to 1000° C. under an oxidizing atmosphere that contains oxygen atoms more than the number of moles of the supported platinum.
As the platinum is thus heated under the oxidizing atmosphere, platinum oxides appear in the surface of each platinum particle that has grown in size in use. Because platinum oxides have a high interactivity with rare earth oxides, the platinum oxides in the surface of the platinum particle move to the surface of the carrier, and thus platinum is exposed at the surface of the platinum particle. The exposed platinum is then oxidized by oxygen into platinum oxides, and these platinum oxides move to the surface of the carrier in the same manner described above. As this happens repeatedly, the platinum particles supported on rare earth oxide, or the like, are gradually dispersed in the surface of the carrier and their particle sizes decrease, thus allowing platinum oxides to be dispersedly supported on the carrier. When a catalyst in which platinum oxides are dispersedly supported on the carrier as mentioned above is placed in contact with an exhaust gas having a stoichiometric atmosphere or a reducing atmosphere, the platinum oxides are reduced into platinum, whereby the exhaust gas purification catalyst is recovered. Note that platinum oxides are easily reduced and thus their reduction reactions tend to easily occur.